Logical fallacies > Unwarranted extrapolation

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The tendency to make huge predictions about the future on the basis of a few small facts is a common logical fallacy.

As Stuart Chase points out, "it is easy to see the persuasiveness in this type of argument. By pushing one's case to the limit... one forces the opposition into a weaker position. The whole future is lined up against him. Driven to the defensive, he finds it hard to disprove something which has not yet happened.

Extrapolation is what scientists call such predictions, with the warning that they must be used with caution. A homely illustration is the driver who found three gas stations per mile along a stretch of the Montreal highway in Vermont, and concluded that there must be plenty of gas all the way to the North Pole. You chart two or three points, draw a curve through them, and extend it indefinitely."
(Chase, 1952)

This logical sleight of hand often provides the basis for an effective fear-appeal. Consider the following contemporary examples:

  • If Congress passes legislation limiting the availability of automatic weapons, America will slide down a slippery slope which will ultimately result in the banning of all guns, the destruction of the Constitution, and a totalitarian police state.
  • If the United States approves NAFTA, the giant sucking sound that we hear will be the sound of thousands of jobs and factories disappearing to Mexico.
  • The introduction of communication tools such as the Internet will lead to a radical decentralization of government, greater political participation, and a rebirth of community.

When a communicator attempts to convince you that a particular action will lead to disaster or to utopia, it may be helpful to ask the following questions:

  • Is there enough data to support the speaker's predictions about the future?
  • Can I think of other ways that things might turn out?
  • If there are many different ways that things could turn out, why is the speaker painting such an extreme picture?